Add mul_add_relaxed methods for floating-point types

[landsharkiest]

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Implements mul_add_relaxed for f16, f32, f64, and f128, which computes (self * a) + b with relaxed precision semantics. Unlike mul_add which guarantees a fused operation, this variant allows the compiler to choose between fused or separate operations based on target performance.

This fills the gap between the precision-guaranteed mul_add and the fully-optimizable algebraic operators, providing target-specific optimization while maintaining reasonable floating-point semantics.

Tracking issue: #151770

try-job: dist-i586-gnu-i586-i686-musl

[rustbot]

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[RalfJung]

@landsharkiest friendly reminder that this PR is waiting for you to react to the reviewer's feedback and to fix CI. :)
if you no longer have time to work on this, that's fine -- just let us know so the task can be picked up by someone else.

[folkertdev]

Another thing, please update the doc comment of the intrinsics to point to the public methods, like here

/// The stabilized version of this intrinsic is 
/// [`f128::mul_add`](../../std/primitive.f128.html#method.mul_add) 

rust/library/core/src/intrinsics/mod.rs

Lines 1459 to 1480 in a6050b7

	/// Returns `a * b + c` for `f128` values.
	///
	/// The stabilized version of this intrinsic is
	/// [`f128::mul_add`](../../std/primitive.f128.html#method.mul_add)
	#[rustc_intrinsic_const_stable_indirect]
	#[rustc_intrinsic]
	#[rustc_nounwind]
	pub const fn fmaf128(a: f128, b: f128, c: f128) -> f128;
	/// Returns `a * b + c` for `f16` values, non-deterministically executing
	/// either a fused multiply-add or two operations with rounding of the
	/// intermediate result.
	///
	/// The operation is fused if the code generator determines that target
	/// instruction set has support for a fused operation, and that the fused
	/// operation is more efficient than the equivalent, separate pair of mul
	/// and add instructions. It is unspecified whether or not a fused operation
	/// is selected, and that may depend on optimization level and context, for
	/// example.
	#[rustc_intrinsic]
	#[rustc_nounwind]
	pub const fn fmuladdf16(a: f16, b: f16, c: f16) -> f16;

We use the prasing of "stabilized" here even if the methods are not actually stable.

[rustbot]

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

[landsharkiest]

@landsharkiest friendly reminder that this PR is waiting for you to react to the reviewer's feedback and to fix CI. :) if you no longer have time to work on this, that's fine -- just let us know so the task can be picked up by someone else.

Sorry got caught up in school and job searching, will finally put this away! :)

[rust-bors]

❗ The PR has only one commit.

[tgross35]

Oi and it looks like I can't push locally. @landsharkiest sorry about that, looks like I'll need you to fix the commit message.

@rustbot author

[RalfJung]

That is odd... we should be allowed to push, but somehow it doesn't work. Maybe because of the # in the branch namee I have never seen that before.

[tgross35]

D'oh it's just on my end, need to add --no-verify apparently if you have a pre-push hook set up

@bors r+

[rust-bors]

📌 Commit b533b26 has been tentatively approved by tgross35

It will be put into the queue for this repository once PR CI succeeds.

[RalfJung]

Ah, got it.

... and apparently so did you. 😂

[tgross35]

😆 I won the race but lost the committer field